This invention relates to a racket stringing apparatus used for stringing tennis rackets, racquetball rackets, squash rackets, badminton rackets, and the like.
More particularly, the present invention relates to a racket stringing apparatus in which continuous tension is applied and maintained on the strings as they are being strung by an active driving device, such as a pneumatic system, a hydraulic system, a solenoid or other electromagnetic system, an electric motor, or other actively powered device. This type of apparatus has advantages over currently available racket stringing devices using springs or weights in an attempt to maintain a preset tension on a string.
Typical stringing machines using weights in an effort to maintain the preset string tension are disclosed, for example, in U.S. Pat. Nos. 2,188,250 of Serrano, issued Jan. 23, 1940, 2,246,109 of Serrano, issued June 17, 1941, 2,389,609 of Caro et al., issued Nov. 27, 1945 and 4,130,278 of Gutzwiller, issued Dec. 19, 1978. Machines in which the tension is to be maintained by weights are disadvantageous because they are bulky and cumbersome to use and it takes a comparatively long time for the string to be elongated and maintained in the elongated condition at the appropriate tension.
Typical examples of machines using springs in an attempt to maintain the proper preset string tension are those disclosed in U.S. Pat. Nos. 2,043,813 of Saladino, issued June 9, 1936, 2,091,654 of Roberts et al., issued Aug. 31, 1937, 2,154,870 of Serrano, issued Apr. 18, 1939, 3,441,275 of Held, issued Apr. 29, 1969, 3,823,609 of Miyagawa et al., issued July 16, 1974, and 4,249,732 of Balaban, issued Feb. 10, 1981.
The uniformity of the tension which can be applied to racket strings springs in these machines is affected by such factors as spring wear, wear of the spring seats, and temperature.
It is believed that racket stringing devices in which the tension on the string is maintained by an active or driven tensioning device, as opposed to passive tensioning devices, such as springs or weights, provide a more uniform, positive and accurate tensioning of the string and are able to maintain the tension on the string more effectively than the passive tensioning devices. Examples of stringing apparatus using active tensioning means include U.S. Pat. Nos. 2,100,948 of Doll, issued Nov. 30, 1937, 2,309,849 of Kausal et al., issued Feb. 2, 1943, 3,635,080 of Krueger, issued Jan. 18, 1972, and 3,913,912 of Smith, issued Oct. 21, 1975. U.S. Pat. No. 1,877,787 of Argabrite, issued Sept. 20, 1932, shows the use of a plurality of hydraulic or pneumatic cylinders movably mounted on several bases at the corner of a leather stretching apparatus. This patent is not believed to be analagous to the present invention, but is mentioned for the purpose of complying with the requirements for a full disclosure.
The hydraulically driven or pneumatically driven stringing machines known to the inventor are generally bulky, difficult to assemble and manufacture and are complicated in operation.
The present invention has been designed to overcome the problems of the prior art and to allow even unskilled racket stringers to produce properly strung rackets having uniform string tension using apparatus which is easy to operate and which uniformly and continuously maintains a predetermined tension on a string during the entire racket stringing process.
The present invention includes performance characteristics which have the following advantages: minimization or elimination of operator error, racket frame distortion, string elongation in a strung racket, frictional losses of string on string and string on the frame, off-center pulling of the string, structural deflection of the stringing apparatus and the string clamps, and lack of uniformity and repeatability of string tensioning.
The present invention also has the advantages that it will not mar or scratch the racket frame, allows maximum string hole access, adaptability to a racket frame head having any radius of curvature, allows sufficient frame flexure to accomodate the tension applied to the frame head by the string during the stringing process, allows rapid frame mounting and removal, and other advantages which will be apparent to those skilled in the art after considering the following description.
The present invention broadly includes several novel structures, systems and features for achieving the desired result of properly strung rackets having the proper string tension and which are capable of maintaining the desired string tension because of the way the rackets were strung.
One such system is the tensioning system which provides an active tensioning drive means which is actuated automatically after a sufficient initial tension is placed on the string. A brake locks a portion of a tension head assembly and cooperates with the drive means for applying and maintaining a predetermined tension on the string.
Another novel feature of the present invention is the racket frame clamp assembly. The racket frame clamp assembly of the present invention includes adjustable frame clamp support arms carrying the frame clamps to allow the clamps to be self-centering with respect to the vertical axis of the base of the stringing apparatus. This vertical axis is designed to correspond with the center of the racket head so that minimum frame distortion and string friction occur, while at the same time providing maximum control and convenience for the operator, including allowing the operator to slightly prestress the unstrung racket frame.
Still another novel structural system developed for the present invention relates to the racket frame head clamp adjacent the tip portion of the racket. This structure provides for positive retention of a racket frame and further minimizes head distortion during stringing. It enables the racket to be firmly, yet quickly clamped in and removed from the racket frame clamp assembly.